Techniques for hydraulically fracturing subterranean formations by injecting a fracturing fluid down a well and into the formation under sufficient pressure to create fractures in a formation are well known. Proppant materials are generally entrained in the fracturing fluid and are deposited in the fracture to keep the fracture open.
After fracturing the formation, hydrocarbonaceous fluids are produced from the formation into the well. These produced fluids may carry sand entrained therein, particularly when the subsurface formation is as unconsolidated formation. Produced sand is undesirable for many reasons. It is abrasive to components found within the well, such as tubing, pumps and valves, and must be removed from the produced fluids at the surface. Further, produced sand may partially or completely clog the well, subsequently inhibiting production thereby making necessary an expensive workover. In addition, sand flowing from the subsurface formation may leave therein a cavity which may result in caving of the formation and collapse of a well casing.
Often after completion of hydraulic fracturing, a steam-flood or other heat generating method is used to heat the formation to remove hdyrocarbonaceous fluids therefrom after having placed a proppant into the created fractures. Proppants utilized in this manner keep the created fractures from closing. They also assist in reducing undesired fines from being carried from the formation with the produced fluids. Also, proppants increase the permeability thereby allowing more intimate contact of the heating medium with the formation. Generally sand is used as a proppant.
Based on recent research concerning proppant slurry transport in hydraulic fractures, it has become more apparent that density effects of slurry volume are important in the deposition of proppant material used in a treatment to hold the fracture open. During the fracturing treatment, a neat fluid i.e., fracturing fluid without proppant therein, is used to create a hydraulic fracture.
Under fracturing pressure, following the creation of the initial fracture, a fracturing fluid containing a proppant therein, is injected into the created fracture. Because the proppant is added to the slurry in increasing concentrations, the effective density of the slurry is greater. Therefore, proppant has a tendency to fall to the bottom of the created fracture. The phenomena related to this type of proppant settling has been called "convection."It is described in SPE paper 24825 authored by M. P. Cleary and A. Fonseca. This paper was presented at the 67th Annual Technical Conference and Exhibition of the SPE. It is entitled "Proppant Convection and Encapsulation in Hydraulic Fracturing: Practical Implications of Computer and Laboratory Simulations".
Therefore, what is needed is a method for effective proppant placement during fracturing which will diminish the "convection effect" so as to allow more effective proppant deposition in the main part of the created fracture.